Hybrid organic-inorganic nanocomposites of polymers and clay platelets represent a type of advanced material with promising applications in biosensing and catalysis, as well as in the fuel cell and pharmaceutical industries, including applications involving the controlled release of drugs. These materials also have superior gas barrier functions and are useful as fire retardants.
Clay is an inexpensive material that is chemically and thermally stable. Clay nanosheets interact with synthetic polymers, organic cations, amino acids and proteins. Because of the high aspect ratio and high surface-to-volume ratios of clay nanosheets, the addition of small amounts (e.g., less than about 10% by weight) of a clay such as montmorillonite to polymer materials significantly improves the mechanical and thermal properties of the resulting nanocomposites. Positively-charged polymers and/or cationic surfactants can adsorb at the silicate surfaces of the clay via electrostatic interactions with negative charges at the clay basal plane. Moreover, organo-modified clays are broadly used in the preparation of nanocomposites, allowing introduction of specific functionalities into the nanocomposite materials via an appropriate selection of molecules adsorbed to the clay. Recently, attention has been attracted by advanced functional stimuli-responsive materials constructed from organic and inorganic components, in response to a growing demand to deliver certain molecules at specific conditions when and where their activities are desired.
Often, delivery of functional molecules may be realized in the vicinity of modified surfaces, such as a surface of a biomedical device or a scaffold. The layer-by-layer (LbL) technique is a powerful tool of surface modification enabling construction of multifunctional thin-film surface coatings. The technique is based on sequential adsorption of oppositely-charged or hydrogen-bonded molecules at a substrate surface, and allows control of the thickness, structure and properties of the coatings. The LbL technique allows the use of aqueous solutions, which is environmentally attractive and enables incorporation of charged and chargeable molecules within LbL films. Moreover, multilayer films can be fabricated on substrates of different shapes. The potential of polyelectrolyte multilayers for biomedical applications has been discussed in recent reviews.
One useful constituent of LbL films is the clay nanosheet or platelet. In prior art embodiments of LbL films, positively-charged polymers are paired with negatively-charged polymers, or negatively-charged clay platelets are paired with positively-charged polymers. The clay and polymers bond to each other through electrostatic interactions. Another reported scenario is binding of negatively-charged clay platelets with neutral polymers (i.e., polymers having no charges at any pH) through hydrogen bonding. For example, U.S. Published Patent Application No. 2004/0053037 discloses construction of LbL barrier films containing negatively-charged inorganic materials and organic materials comprising cationic polyelectrolytes or hydrogen bonding neutral polymers (e.g., a homopolymer of acrylamide or a polyvinyl alcohol copolymer).
Clay-containing LbL films are known for their high mechanical strength, as well as for their antiflammability and oxygen barrier properties. For example, U.S. Pat. No. 7,045,087 describes LbL clay-containing films having sufficient mechanical stability to be free-standing. Such films were assembled by combining layers of positively-charged polymers and negatively-charged clay platelets.
It is also known that clay-containing LbL films perform well as gas, flavor and aroma barriers. In other applications, the inclusion of electrochemically active corrosion inhibitors, render LbL organic-inorganic assemblies useful for corrosion protection of metal surfaces. U.S. Published Patent Application No. 2003/0027011 describes the preparation of clay-containing films by alternating layers of poly(dimethyldiallylammonium chloride) (PDDA, which is a positively-charged polymer), and clay, as well as films prepared from layers of PDDA, poly(acrylic acid) (PAA) and clay, wherein the PAA and clay layers were alternated as the negatively-charged species.
The majority of clay-containing films are designed for use in the dry state. While the use of clay films as diffusion barriers in water have been reported (see, A. Zhuk et al., ACS Nano 5 (2011) 8790-9, which is incorporated by reference herein). However, such films were not two-component clay-polyanion films.